WIP: Asynchronous send #399

Job	Run time
test (1.24.x)	1m 40s
golangci-lint	37s
test (stable)	1m 53s
build-upload (amd64, windows, windows)	1m 27s
build-upload (amd64, mac, darwin)	1m 21s
build-upload (amd64, linux, linux)	1m 25s
	8m 23s

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